GHRP-2 vs GHRP-6: The Original Hexapeptides Compared

One programme, two peptides

GHRP-6 and GHRP-2 came out of the same laboratory. Cyril Bowers and colleagues described GHRP-6, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, in 1984 as a synthetic hexapeptide that made the pituitary release growth hormone without touching LH, FSH, TSH or prolactin. It worked in rats, monkeys, lambs and calves, by intravenous, subcutaneous and intraperitoneal routes, and GH peaked 10 to 20 minutes after an IV dose.

GHRP-2, D-Ala-D-2Nal-Ala-Trp-D-Phe-Lys-NH2, is a later design from the same programme, built for more potency. Its generic name is pralmorelin. Neither peptide's receptor had a known natural partner until 1999, when ghrelin was identified as the stomach hormone that uses it. Both GHRPs are, in effect, small synthetic ghrelin mimics, which explains everything that follows: the GH pulse, the hunger, and the cortisol.

Side by side

What the human studies measured

GHRP-6 in healthy men (Bowers 1990). Eighteen men received single IV doses of 0.1, 0.3 and 1.0 mcg/kg. Mean peak GH climbed from 1.2 mcg/L on placebo to 7.6, 16.5 and 68.7 mcg/L. Low doses combined with GHRH released GH synergistically, which is where the whole idea of pairing a GHRP with a GHRH analogue comes from. The only side effect noted was brief facial flushing, and it occurred in the men who received GHRH, not GHRP-6. At the highest GHRP-6 dose, prolactin and cortisol roughly doubled.

GHRP-6 pharmacokinetics (Cabrales 2013). A Cuban group gave nine healthy men single IV boluses of 100, 200 and 400 mcg/kg. The peptide followed a two-phase decline, with a distribution half-life of about 7.6 minutes and an elimination half-life of about 2.5 hours. Four of the nine men showed unexplained concentration spikes during the elimination phase.

GHRP-2 as a diagnostic (Chihara 2007). Japanese researchers gave 100 mcg IV to 77 healthy adults and 58 adults with confirmed GH deficiency. GH peaked within 60 minutes in every subject. Healthy subjects reached a mean peak of 84.6 mcg/L; patients reached 1.36. That gap, and the fact that the test was reproducible and only slightly affected by age and body fat, is why GHRP-2 is a licensed diagnostic in Japan. The insulin tolerance test it replaces carries a risk of severe hypoglycaemia.

GHRP-2 in children (Mericq 1998). Six prepubertal children with GH deficiency received daily subcutaneous GHRP-2 at rising doses over eight months. Overnight GH secretion rose with dose and growth velocity improved during treatment, but IGF-1 and IGFBP-3 did not increase, and each injection's effect was short. No side effects or toxicities were observed. The authors concluded a longer-acting form would be needed for therapy.

The pattern across both peptides: reliable, short GH pulses, well characterised as tests, and almost no data on what months of use does to a healthy adult's body composition. No trial has measured that for either.

Potency, efficacy and what "stronger" means

The clearest direct comparison is in pigs, from the 1998 paper that introduced ipamorelin. GHRP-2 needed a smaller dose for a half-maximal effect (higher potency) but produced a lower maximum GH level (lower efficacy) than GHRP-6. So "GHRP-2 is stronger" and "GHRP-6 releases more GH" are both defensible, depending on which parameter you mean. In people, no trial has put the two head to head, and dose-for-dose claims online are extrapolation.

The same pig study is the source of the cortisol comparison: both GHRPs raised ACTH and cortisol, and ipamorelin did not, at doses more than 200 times the amount needed for GH release. That is why ipamorelin was developed, and why it is the more common choice locally today.

Hunger, prolactin, cortisol

Because both peptides act at the ghrelin receptor, appetite is a class effect. GHRP-6 has the stronger reputation for it, but a 2017 review of the GHRP literature describes GHRP-2 raising food intake in cancer-bearing rodents. The 1990 human study documents the prolactin and cortisol rise for GHRP-6 at 1 mcg/kg; the pig work documents it for both. Whether these matter over months of use has not been studied, since no such study exists.

Any compound that raises GH and IGF-1 carries the theoretical long-term concerns that attach to growth hormone itself: fluid retention, joint aches, reduced insulin sensitivity, and the open question of IGF-1 and cancer risk. The US FDA's compounding safety list includes both GHRPs, citing immunogenicity risk and, for GHRP-2, adverse event reports that include increased insulin requirements and pancreatitis. The side effects hub collects the GH-axis picture in one place.

Where each one went

GHRP-2 became a diagnostic drug in Japan. GHRP-6 became a research tool and, in Cuba, the subject of a long line of tissue-protection studies in animals that led to a 2024 phase I/II stroke trial in which it was always given together with EGF, so its own contribution cannot be separated. Neither became a treatment. The class as a whole was overtaken by ipamorelin for selectivity and by the oral MK-677 for convenience; for the wider picture see HGH vs peptides.

Status in the Philippines

Neither GHRP is registered with FDA Philippines, so neither can be sold here as a medicine. Both circulate through research-chemical suppliers. For anyone competing under the World Anti-Doping Code the position is unambiguous: GHRP-2 and GHRP-6 are both named under S2 and prohibited at all times, and anti-doping laboratories have published urine detection methods.

Storage

Both are small, fairly stable hexapeptides, but heat is still their enemy. Keep unmixed vials in the fridge rather than on a shelf through a Manila summer, and at 2 to 8°C once reconstituted with bacteriostatic water. The reconstitution calculator handles the mixing arithmetic.